Connection structure for metallic members and connecting method therefor

ABSTRACT

Sn—Zn alloy is plated on at least one of a first metallic member and a second metallic member, Sn—Ag alloy is used as a solder, the first metallic member and the second metallic member are connected by the solder, and a connection structure of metallic members is therefore produced. Heat degradation of plating and the contact corrosion between solder and plating can thereby be prevented, solderability can be improved, and qualities such as corrosion resistance and connecting strength at the connection structure can be improved.

TECHNICAL FIELD

The present invention relates to a connection structure for metallicmembers, which is, for example, suitable as a connection structurebetween a tank and a pipe of a fuel tank for automobiles, etc., and inparticular, relates to a technique in which corrosion resistance andsealing can be improved without using Pb.

BACKGROUND ART

For example, in a fuel tank 1 for automobiles, as shown in FIG. 10,pipes such as a filler neck pipe 2 for refueling, a breather pipe 3 forbreathing air in refueling, and a venting pipe 4 for releasing pressurein the fuel tank 1, are connected. In the case in which such pipes areconnected to the fuel tank 1, one end of a pipe P is press-fitted in thefuel tank 1, as shown in FIG. 11, then ring shaped solder S isadjacently placed at a boundary between the pipe P and the fuel tank 1,and this solder S is heated and melted by an electrode 6 for highfrequency induction heating. Then, the melted solder S solidifies at thecorner portion of the boundary between the pipe P and the fuel tank 1,as shown in FIG. 12, and both members are thereby airtightly connected.

As a conventional solder, Pb—Sn alloy has generally been employed.However, it is not desirable to use Pb since there are environmentalregulations on Pb leached from industrial waste such as shredder dust,etc., and substitutes for Pb have been required. Therefore, Ag alloys,Cu or Cu—Zn based alloys, Zn—Al alloys, etc., have recently been used,and in Japanese Unexamined Patent Publication No. 71488/98, a Sn alloysolder (Sn—Ag based) has also been disclosed.

On the other hand, as a material for fuel tanks and pipes, surfacetreated steel sheets which are subjected to Zn plating, Al alloyplating, Zn alloy plating, etc., may be used. Alternatively,after-treatment plating which is carried out after material is processedfrom a steel plate, may be carried out. In any case, a fuel tank 1 and apipe P on which are formed platings M1 and M2, are connected together bysolder S, as shown in FIG. 12. The solder S is applied to suchconnections since the heating temperature thereof is lower than those ofother methods such as welding, etc., the heat distortion at a positionwhere dimensional accuracy is required can be suppressed in a thinlayer, the sealing thereof is superior, equipment can be miniaturized,etc.

In a fuel tank for automobiles, seals which can withstand high internalpressure are required at soldered portions since internal vapors of fuelexpand with increasing temperature, and in addition, reliability anddurability in which functions thereof are not damaged by vibration andacceleration during driving of the automobile, are also required. Thefuel tank is often installed under the floor of the car body, andtherefore, a high level of coating and high corrosion resistance arerequired, even at the soldered portions, since they are exposed tosevere road environments and climatic conditions, such as snowmelt salt,mud, water, humidity, are splashed gravel. Furthermore, corrosionresistance of the inner surface is also required, since corrosivecomponents such as acids and peroxides are produced when gasolinedecomposes in a fuel tank.

However, there was a problem in that surrounding plating is heated byheating during soldering, and the plating is thereby heat-deteriorated.That is, it is necessary to heat to a temperature 50° C. or more(preferably 100° C.) higher than the melting point of solder, in orderto securely fix members by increasing the solderability (wettability).In particular, a pipe wall portion just above a high frequency heatingelectrode is heated to a high temperature by this heating and platingmetal is alloyed with Fe material, and the corrosion resistance isthereby lowered and the plating is weakened thereby. Depending onconditions, a plating M2 may melt and flow down, as shown in FIG. 13(a),or a porous oxide film M3 may be formed by oxidizing the plating M2, asshow in FIG. 13(b), and corrosion resistance is thereby drasticallylowered, and the application property is also lowered in the lattercase.

In addition, in the case in which the solder material is different fromthe plating material, there was a problem in that the corrosionresistance is lowered by contact corrosion occurring in which the basemetal acts as an anode. Therefore, it is an object of the presentinvention to provide a connection structure for metallic members and aconnecting method in which high reliability and high durability can beensured even in use under severe conditions. Specifically, in thepresent invention, plating and solder materials are selected inconsideration of following points.

{circumflex over (1)} A solder having superior solderability to platingis selected in order to obtain a connection structure having superiorqualities in which a soldered portion thereof has high strength andthere are few internal defects.

{circumflex over (2)} Materials for the solder and the plating havingsmall differences in corrosion potential are selected in order to reducecontact corrosion between the solder and the plating.

{circumflex over (3)} A plating having high corrosion resistance tosaltwater or decomposed gasoline is selected in order to improve thecorrosion resistance of the plating.

{circumflex over (4)} A solder having a low melting point is selected inorder to reduce heat deterioration of the plating caused by heatingduring soldering and to increase thereby adhesion of films. Inparticular, it is desirable that melting points of the solder and theplating be 180° C. or more in view of the fact that baking finishing iscarried out at 150° C. or more in a subsequent process.

DISCLOSURE OF THE INVENTION

(1) Prevention of Heat Deterioration of Plating

The present inventors researched materials for plating and solder fromthe above points of view. Firstly, melting points of various alloys ormetals were enumerated in the following, in order to take into accountthe heat deterioration prevention of the plating. In comparison withproperties shown in Table 1, Zn to Cu—Zn based alloy (lines 6 to 10)have high melting points, and it is therefore anticipated that heatdeterioration, such as oxidation, etc., of the plating material (Pb—Snalloy, etc.) will occur.

TABLE 1 Melting Evalua- Alloy Names Symbols Points Primary Uses tionLead-tin alloy Pb—Sn 200 Soldering, Plating Good Tin-silver alloy Sn—Ag215 Soldering Good Tin-antimony alloy Sn—Sb 220 Soldering Good Tin-zincalloy Sn—Zn 230 Plating Good Zinc Zn 420 Plating Bad Zinc-aluminum alloyZn—5Al 361 Soldering, Plating Bad Aluminum-silicone Al—Si 585 PlatingBad alloy Silver-copper alloy Ag—Cu 780 Brazing Bad Copper-zinc alloyCu—40Zn 890 Brazing Bad

(2) Corrosion Resistance of Plating

In a fuel tank for automobiles, corrosion resistance to the externalenvironment and corrosion resistance to acids and peroxides produced bythe degradation of fuel are required. Then, with respect to the internalcorrosion resistance which prevents Fe in a saltwater environment fromcausing corrosion and the external corrosion resistance which hasresistance to gasoline degradation products including formic acid oracetic acid, the evaluations of various metals are described in Table 2.As is apparent from Table 2, Al—Si alloy and Sn—Zn alloy are preferableas a plating material.

TABLE 2 Plating Metal Zn Zn—Ni Al—Si Sn—Zn Sn Cu Ag Internal Bad AverageGood Good Bad Bad Bad corrosion resistance External Bad Bad Good GoodGood Bad Good corrosion resistance

(3) Contact Corrosion Resistance and Solderability

The order of corrosion potential of various metals in a saltwater isshown in FIG. 1. In the case in which two metals shown in FIG. 1 areemployed for soldering and plating, if the locations of the corrosionpotential of the two metals are far apart, the difference in thecorrosion potentials is high, and the base metals are easily corroded.According to this rule as a standard, the contact corrosion resistancesof the combinations of various metals were evaluated and the results areshown in Table 3. Additionally, with respect to each combination ofmetals, the solderabilities thereof were also evaluated. In thisevaluation, the following criteria were used: cases where thesolderability thereof was the same as that of combination of Pb—Sn alloyplating and Pb—Sn alloy solder are indicated by ∘, cases where it wasslightly inferior to the above, but was at an allowable level areindicated by Δ, and cases where soldering thereof was difficult orimpossible are indicated by ×, and these results are shown in Table 3.

TABLE 3

As is apparent from Table 3, the combination of Sn—Zn alloy plating andsolder was best in the contact corrosion resistance. However, thiscombination has inferior solderability and would be difficult to use inpractice because zinc oxide is a product on the surface of melted metalin soldering. In contrast, the combination of Sn—Ag alloy solder andSn—Zn alloy plating has superior solderability, and the contactcorrosion resistance is also at an allowable level. In addition, thesealloys also have low melting points and superior internal and externalcorrosion resistances.

Therefore, the present invention has been made based on the above testsand is characterized in that in a connection structure for metallicmembers in which a first metallic member and a second metallic memberare connected by solder, Sn—Zn alloy is plated on at least one of thefirst and second metallic members, and the solder consists of Sn—Agalloy.

In the connection structure for the metallic member as composed above,solder and plating easily melt each other since the melting pointsthereof are close, and thus, the solderability is superior, the numberof internal defects is small, and the metallic members are firmlyadhered. A seal which resists high internal pressure in the interior ofa fuel tank and which has reliability and durability and are not damagedby vibration and acceleration in the driving of automobiles can therebybe obtained. In contrast, in the case in which the difference betweenthe melting points of solder.and plating is large, it is necessary toconform the heating temperature to a higher melting point. Therefore,the metal having a lower melting point is oxidized by heating and oxidefilm is formed, and Fe, which is base, is thereby easily corroded andthe adhesion of the films is decreased. However, such problems do notoccur in the present invention. Additionally, the internal corrosionresistance and the external corrosion resistance are superior since theplating consists of Sn—Zn alloy, and generation of contact corrosion isalso reduced since differences in corrosion potentials are small.

Here, a Zn-rich layer in which Sn—Zn alloy plating is alloyed with thesolder is desirably provided on the surface.of a portion of which thesolder and the plating melt with each other. By providing the Zn-richlayer, contact corrosion between the plating and the solder isprevented, and a chemical conversion coating is easily formed inpre-treatment processing and the adhesion of films is thereby improved.

In addition, the Sn—Zn alloy plating desirably has a composition of 93to 55% by weight of Sn and 7 to 45% by weight of Zn. When the content ofZn is below 7% by weight, the amount of Zn to prevent corrosion of Fe islow, and Fe is thereby easily corroded, and the corrosion resistance ina saltwater environment is decreased. In contrast, when the content ofZn exceeds45% by weight, Zn oxide in a porous state is formed on thesurface of a portion of which the solder and the plating melt with eachother. Therefore, the solderability deteriorates and the fasteningstrength is decreased.

Furthermore, a connecting method for metallic members, according to thepresent invention, is characterized in that a connecting method formetallic members in which a first metallic member and a second metallicmember are connected by solder, comprises plating Sn—Zn alloy on atleast one of the first and second metallic members, using Sn—Ag alloy asa solder, and connecting the metallic members while cooling at theconnected portion. According to the present invention, formation ofporous layers by washing away the plating due to overheating or byoxidation thereof can be reliably prevented. In particular, when onemetallic member is a hollow member such as a pipe, a moderate suitablecooling effect can be obtained preferably by supplying cooling gas suchas air or other gas into the inside of the hollow member. In addition,the soldering conditions have been controlled by electric power suppliedto an electrode for high frequency heating heretofore. However, electricpower control has recently been further expanded by further addingcooling thereto, and the control has become easy and the quality thereofhas also been stabilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the order of corrosion potential withrespect to various metals.

FIGS. 2A to 2D are cross sectional views showing soldered portions indetail.

FIG. 3 is a graph showing relationships between Zn content and Agcontent on the surface of a soldered portion.

FIG. 4 is a graph showing relationships between Zn content, the pipepulling strength, and the rust occurrence cycle, respectively.

FIGS. 5A to 5C are longitudinal sectional views showing cooling methodsfor soldered portions, respectively.

FIG. 6 is a longitudinal sectional view showing temperature measuringpoints in a soldered portion.

FIG. 7 is a graph showing temperatures of each point in a solderedportion.

FIG. 8 is a graph showing relationships between electric power duringheating and temperatures in a soldered portion or a plated portion.

FIG. 9 is a graph showing controlling ranges of electric power forheating in the case in which a soldered portion is cooled and the casein which it is not cooled.

FIG. 10 is a perspective view showing a fuel tank.

FIG. 11 is a perspective view showing a state in which a pipe issoldered to a fuel tank.

FIG. 12 is a longitudinal sectional view showing a soldered portion indetail.

FIGS. 13A and 13B are longitudinal sectional views showing inferiorqualities which occur in soldered portions.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention is explained referring to FIG. 2 in moredetail. As shown in FIG. 2A, a fuel tank or container 1 (first metallicmember) has Sn—Zn alloy plating M1 plated on the internal and externalsurface of raw material 1 a made of Fe. In this fuel tank 1, a pipe(second metallic member) P in which Sn—Zn alloy plating M2 is plated onthe internal and external surfaces is inserted under pressure. In thefigures, description of internal plating is omitted. A ring of solder Sis fitted on the pipe P, and the solder S is heat-melted by an electrodefor high frequency heating (not shown) displaced in close to the solderS. FIGS. 2B and 2C show conditions in which the solder S is solidified.As shown in FIG. 2C on the surface of the solder S, the plating M1 andthe plating M2 are alloyed to the solder S, and a Zn-rich layer R isformed. Next, as If shown in FIG. 2D, the fuel tank 1 and the pipe P arecoated, preferably after carrying out the pretreatment process, and theyare covered with a coating film C. In such a connection structure formetallic members, the contact corrosion between the plating and thesolder is prevented since the Zn-rich layer R is formed on the surfaceof the solder S. and the adhesion of films is superior because achemical conversion coating is easily formed in the pre-treatmentprocess. Therefore, the corrosion resistance on the surface of thesolder S can be drastically improved.

As a solder according to the present invention, a solder consisting of94 to 98% by weight of Sn and 2 to 6% by weight of Ag is preferable, andthird addition metals such as Zn, Cu, and Bi may be contained in anamount below 3% by weight. Furthermore, the present invention isachieved-by Sn—Zn alloy plating being plated on at least one of thefirst and second metallic members. With respect to the other metallicmember, Ni plating may be plated thereon instead of the Sn—Zn alloyplating, or nothing may be plated thereon. It is desirable that thecontent of Zn in the Sn—Zn alloy plating be 7 to 45% by weight. When thecontent of Zn is below 7% by weight, the amount of Zn for preventingcorrosion of Fe is small, and Fe is thereby easily corroded and thecorrosion resistance in a saltwater environment is decreased. Incontrast, when the content of Zn exceeds 45% by weight, Zn oxide in aporous state is formed at regions shown by symbol Z in FIG. 2C of thesolder S. Therefore, the solderability is deteriorated and the fasteningstrength is decreased. Additionally, the thickness of the Sn—Zn alloyplating is desirably 3 to 13 μm. Furthermore, if a chromate-treatedfilm, an organic coating film having a thickness of 1 μm or less, or aninorganic composite coating film is provided on the surface of the Sn—Znalloy plating, the corrosion resistance is further improved.

EXAMPLES

In the following, the present invention is explained referring toExamples in more detail.

1. First Embodiment

A. Preparation of Samples

A steel pipe (member A) having an outer diameter of 16 mm and an innerdiameter of 14 mm in which Sn—Zn alloy plating or Ni plating was platedon the internal and external surfaces and a steel plate (member B)having a thickness of 1 mm in which Sn—Zn alloy plating was plated onthe surface and the rear surface thereof, were prepared. A connectionstructure of the Example, as shown in FIG. 13, was obtained by making ahole on the steel plate, inserting the pipe into the hole underpressure, and connecting together using a ring of Sn—Ag alloy solder. Inaddition, a connection structure of the Comparative Example was obtainedin the same manner as that of Example, except that the platingcomponents of the members A and B were changed into a component otherthan Sn—Zn alloy. Each connection structure was coated all over thesurface so as to have a thickness of 20 μm. The types of solder andplating and contents of Ag and Zn (% by weight) in each connectionstructure are described in Table 4.

TABLE 4 Solder Member A Member B Example 1 Sn—Ag (Ag: 3.5%) Sn—Zn Sn—Zn(Zn: 7%) (Zn: 7%) Example 2 Sn—Ag (Ag: 3.5%) Sn—Zn Sn—Zn (Zn: 8%) (Zn:30%) Example 3 Sn—Ag (Ag: 3.5%) Sn—Zn Sn—Zn (Zn: 30%) (Zn: 30%) Example4 Sn—Ag (Ag: 3.5%) Sn—Zn Sn—Zn (Zn: 45%) (Zn: 45%) Example 5 Sn—Ag (Ag:3.5%) Ni Sn—Zn (Zn: 8%) Example 6 Sn—Ag (Ag: 3.5%) Sn—Zn Sn—Zn (Zn: 55%)(Zn: 55%) Comparative Sn—Ag (Ag: 3.5%) Ni Ni Example 1 Comparative Sn—Ag(Ag: 3.5%) Zn Zn—Ni Example 2

B. Test

With respect to each connection structure, a combined corrosion testbased on automobile standard (JASOM 610-92) was carried out and thecorrosion resistance thereof was examined. In this combined corrosiontest, {circumflex over (1)} NaCl aqueous solution at 35° C. was sprayedon the connection structure for 2 hours, {circumflex over (2)} this wasdried for 4 hours in air at 60° C. and a relative humidity of 20 to 30%,and {circumflex over (3)} this was left for 2 hours in a moistenvironment at 50° C. and a relative humidity of over 95%. The processof {circumflex over (1)} to {circumflex over (3)} was defined as 1cycle, and the number of cycles until red rust was generated at theconnection structure was counted.

The pipe was pulled out upward of the steel plate in the state shown inFIG. 12, and the pulling load of the pipe was measured. Additionally,each condition of the solder before coating was observed by visualobservation, and the solderability thereof was evaluated. The aboveresults are described in Table 5. In the evaluation of thesolderability, the following criteria were used: cases where it had thesame solderability as that of a combination of Pb—Sn alloy plating andPb—Sn alloy solder which is most generally used are indicated by ⊚,cases where it was slightly inferior to the above, but was preferableare indicated by ∘, cases where it was inferior, but was at an allowablelevel are indicated by Δ, and cases where soldering thereof wereunacceptable are indicated by ×.

TABLE 5 Zn Pipe Corrosion Content (wt %) Pulling Resistance MemberMember Load Solder- (Number of Cycles A B (kgf) ability until RustAppears) Example 1 7 8 955 ⊚ 22 Example 2 30 8 953 ⊚ 26 Example 3 30 30948 ⊚ 34 Example 4 45 45 880 ◯ 42 Example 5 0 8 950 ⊚ 20 Example 6 55 55600 Δ 47 Comparative 0 0 950 ⊚ 10 Example 1 Comparative 100 94 20 X  5Example 2

As is apparent from Table 5, in connection structures of Examples 1 to3, high pulling loads of about 950 kgf were exhibited and the corrosionresistances were also superior, since the solderabilites were superior.In Examples 4 and 6, since the content of Zn in Sn—Zn alloy plating wasrelatively high, oxides of Zn were produced between solder and platingduring plating, the solderabilities and the pipe pulling loads wereslightly decreased, but there was no problem in practical use.Furthermore, in Example 5, since the plating of member A consists of Niin which the corrosion of Fe cannot be prevented, that is, in which itis a noble metal in comparison with Fe, the corrosion resistance wasslightly decreased, but there was no problem in practical use. InComparative Example 1, the plating did not melt during soldering sinceit was Ni in which the melting point is high, and superior solderabilityand superior pipe pulling load were exhibited. However, the corrosionresistance of Comparative Example 1 was inferior, since the platings ofboth members thereof consisted of Ni. In addition, in ComparativeExample 2, since the content of Zn in the plating was high, the oxide ofZn was formed in the porous state at an alloy layer between solder andplating, and the solderability was extremely deteriorated. Furthermore,in Comparative Example 2, the platings of member A and member Bconsisted of Zn and Zn-Ni for which the order of corrosion potential isquite different. Therefore, contact corrosion was generated between theplating and the members, and the corrosion resistance was more inferior.

FIG. 3 is a graph showing relationships between Zn content and Agcontent on the surface of a soldered portion in Examples 1 to 5 andComparative Example 1. As is apparent from FIG. 3, the larger the Zncontent in the plating, the more Zn-rich the Zn layer formed on thesurface of Sn—Ag alloy solder. Thus, by this Zn-rich layer, contactcorrosion between plating and solder is suppressed and film adhesion isimproved, and the superior corrosion resistance, as described above, canthereby be obtained.

2. Second Embodiment

Connection members in which each Zn content was gradually made to changefrom 0 to 100% by weight were produced in the same manner as the abovefirst embodiment, except for using members A and B plated with Sn—Znalloy having the same Zn content. Next, the pipe pulling load of eachconnection material was measured, and the results are shown in FIG. 4.As is apparent from FIG. 5, the larger the Zn content, the larger thepipe pulling load; however, the pipe pulling load is rapidly decreasedif Zn content exceeds 45% by weight. This occurs because the Zn oxide ina fragile porous state is formed at the alloy layer between solder andplating. As is apparent from this result, it is preferable that the Zncontent in the plating be below 45% by weight.

In addition, with respect to connection structures in which the Zncontent in the plating ranges from 0 to 55% by weight, a combinedcorrosion test was carried out under the same conditions as that of thefirst embodiment, and the results are described in FIG. 4. As isapparent from FIG. 4, when the Zn content is below 7% by weight, thecorrosion prevention function of Zn cannot be obtained, and thecorrosion resistance is rapidly decreased. Therefore, it is preferablethat the Zn content in the plating be 7% by weight or more.

3. Third Embodiment

FIGS. 5A to 5C are drawings showing soldering methods while the pipe iscooled; FIG. 5A shows a method for cooling the inside of the pipe usinga coolant, FIG. 5B shows a method for cooling the outside of the pipeusing a coolant, and FIG. 5C shows a method for dissipating andradiating heat by providing radiating fins on the top of the pipe. Aconnection structure was produced in the same manner as in Example 3 inthe above first embodiment in addition to using the cooling methodsshown in FIGS. 5A to 5C. Then, the cooling effects were examined bymeasuring the temperature at the position shown in FIG. 6 in theconnection structure, the quality (solderability) and the corrosionresistance at the soldered portion were tested by the same method asthat of the first embodiment, and the results are shown in Table 6.

TABLE 6 Cooling Methods (a) Pipe Inside Cooling (b) Pipe Outside Cooling(c) Fin Cooling Coolant a-3 b-2 a-1 a-2 Steam or b-1 Steam or f Air orGas Water Water Mist Air or Gas Water Mist Nothing Cooling Effect ◯ ⊚ ⊚⊚ ⊚ X Solder Quality ⊚ X X X X — Corrosion ⊚ — — — — Δ Resistance(Conventional Property) Total Evaluation ⊚ X X X X X

As shown in Table 6, in the cooling methods of (a-2) and (a-3)overcooling was caused, and incomplete penetration thereby occurred, inparticular, at the interface of pipe and solder, and satisfactory solderqualities were not obtained. In addition, in the cooling method of(b-1), wavy folds due to the coolant were caused on the surface ofunsolidified solder, and in the cooling method of (b-2), cracks due toquenching were generated on the surface of the solder, and satisfactorysolder qualities were not obtained. Furthermore, in the cooling methodof (c) which did not use the coolant, a sufficient effect forcontrolling temperature rise was not obtained. Ultimately, the pipeinternal cooling (a-1) using air or gas as a coolant was most preferablesince a moderate cooling effect was obtained.

FIG. 7 is a graph showing temperatures of each position (A, B, and C)shown in FIG. 6 in the cooling methods of (a-1), (a-3), and (c). Inaddition, the temperature in the case of non-cooling was also describedas a control. As shown in FIG. 7, in the cooling method of (a-3), thetemperature at point A was lowered below the required heatingtemperature (about 340° C.) by overcooling, and the incompletepenetration of solder thereby occurred. Furthermore, in the coolingmethod of (c), there was little difference in temperature from the caseof non-cooling.

Next, combinations of solder and plating shown in Table 7 were soldered,except that cooling methods were changed and connection structures werethereby produced. Then, variously properties of produced connectionstructures were tested, and the results are also described in Table 7.Here, an epoxy-type or melamine-type coating material was coated on theconnection structure after soldering, so as to have a thickness of about20 μm, it was dried for a standard time, and the coating was therebyformed. In addition, the coating was soaked in ion exchanged water at40° C. for 240 hours and was taken out, cross cut patterns 1 by 1 mmwere scored on the surface of pipe by knife, and the film adhesion wasevaluated by peeling cellophane tape from the cross cut patterns.Furthermore, in the evaluation, the following criteria were used: caseswhere the peeling area of each cross cut pattern was 50% or less per onecross cut pattern area, and all of the cross cut pattern satisfied theabove range are indicated by ∘, and cases other than the above cases areindicated by X. “Plating corrosion resistance” indicates the number ofcycles in the combined corrosion test before painting, and “coatingcorrosion resistance” indicates the number of cycles in the combinedcorrosion test after painting.

TABLE 7 Plating Coating Cooling Pipe Pulling Corrosion Film CorrosionPlating Solder Methods Strength (kgf) Solderability Resistance AdhesionResistance Pb—Sn Pb—Sn Nothing 880 ⊚ 3 X 22 Pb—Sn Pb—Sn a-1 870 ⊚ 6 ◯ 42Pb—Sn Pb—Sn a-3 877 X — — — Sn—Zn Sn—Ag Nothing 950 ⊚ 8 X 24 Sn—Zn Sn—Aga-1 955 ⊚ 16 ◯ 60 Sn—Zn Sn—Ag a-3 947 X — — — Zn Sn—Ag Nothing 948 Δ 7 X34 Zn Sn—Ag a-1 950 Δ 16 ◯ 50

As is apparent from Table 7, in the case in which Sn—Zn alloy was usedfor plating and Sn—Ag alloy was used for soldering in the cooling methodof (a-1), extremely superior results were obtained in all properties. Inparticular, it was proven that the pipe pulling strength, the platingcorrosion resistance, and the coating corrosion resistance were moresuperior to the case in which Pb—Sn was used for plating and solderingin the cooling method of (a-1).

Next, with respect to the relationships between heating electric powerin soldering and temperatures of a soldered portion (point, A) and aplated portion (point B) in FIG. 6, the case in which the pipe cooled bythe method of (a-1) and the case in which it was not cooled wereexamined. The results are shown in FIG. 8. As is apparent from FIG. 8,when the heating temperature is below 340° C., incomplete penetration ofsolder occurs. Thus, in order to raise the temperature at point A to340° C. or more, it is necessary that the electric power for heating be1.6 kW or more, irrespective of the performance of cooling. In contrast,when the heating temperature exceeds 500° C., thermal degradation ofplating occurs. Thus, it is necessary that the electric power forheating be 1.7 kW or less in the case of non-cooling. Therefore, in thecase of non-cooling, as shown in FIG. 9, the electric power for heatingmust be controlled ranging from 1.6 to 1.7 kW, that is, within a rangeof 0.1 kW.

In contrast, the temperature at point B after cooling is not veryrapidly increased relative to the increase of the electric power forheating, as shown in FIG. 8. Thus, a temperature at point B can belowered to 500° C. or less, even if the electric power for heating israised to about 1.9 kW. Therefore, in the case of cooling, as shown inFIG. 9, the electric power for heating may be controlled ranging from1.6 to 1.9 kW, that is, within the range of 0.3 kW. This is veryimportant in order to stabilize the qualities. That is, in the case ofsoldering, a target temperature by heating must be controlled at a levelwhich is similar to the lower limit, since the prevention of heatdegradation at plated portions has been regarded as important for sometime. However, when the allowable range of heating temperature isnarrow, incomplete penetration of the solder easily occurs due tovariation of heating temperature, since the temperature at solderedportion (point A) fluctuates greatly depending on the distance betweenan electrode and solder, even if the electric power is the same. Incontrast, according to the present invention, since the allowable rangeof heating temperature is broadened by cooling, the dispersion ofheating temperature of the solder can be absorbed, and the qualities canbe stabilized even by simple control.

Additionally, the present invention is not limited to such a structurecomprising a fuel tank and a pipe, and it can be applied to anyconnection structures for metallic components.

As explained above, according to the present invention, the heatdegradation of plating and the contact corrosion between solder andplating can be prevented, solderability can be improved, and qualitiessuch as the corrosion resistance and connecting strength at theconnection structure can be improved, since Sn—Zn alloy is plated on atleast one of first and second metallic members and Sn—Ag alloy is usedas a solder.

What is claimed is:
 1. A connection structure for metallic members inwhich a first metallic member and a second metallic member are connectedby solder, wherein Sn—Zn alloy is plated on at least one of said firstmetallic member and said second metallic member, and said solderconsists of Sn—Ag alloy.
 2. A connection structure for metallic membersas recited in claim 1, wherein said solder and said plated Sn—Zn alloymelt each other so as to form a melted portion, and a Zn-rich layerformed by alloying said solder and said plated Sn—Zn alloy is providedon a surface of said melted portion.
 3. A connection structure formetallic members as recited in claim 1, wherein said Sn—Zn alloy platingconsists of 93 to 55% by weight of Sn and 7 to 45% by weight of Zn.
 4. Aconnection structure for metallic members as recited in claim 1, whereinsaid solder consists of 94 to 98% by weight of Sn and 2 to 6% by weightof Ag.
 5. A connection structure for metallic members as recited inclaim 1, wherein said solder contains at least one of Zn, Cu and Bi, inthe total amount of 3% by weight or less.
 6. A connection structure formetallic members as recited in claim 1, wherein said plated Sn—Zn alloyhas a thickness of 3 to 13 μm.
 7. A connecting method for a connectionstructure for metallic members, comprising: plating Sn—Zn alloy on atleast one of a first metallic member and a second metallic member; usingSn—Ag alloy as solder; and connecting said first metallic member andsaid second metallic member by said solder while cooling the connectedportion.
 8. A connecting method for a connection structure for metallicmembers as recited in claim 7, further comprising: forming a meltedportion by melting said solder and said plated Sn—Zn alloy with eachother; and forming a Zn-rich layer on a surface of said melted portionby alloying said solder and said plated Sn—Zn alloy.
 9. A connectingmethod for a connection structure for metallic members as recited inclaim 7, wherein said Sn—Zn alloy plating consists of 93 to 55% byweight of Sn and 7 to 45% by weight of Zn.
 10. A connecting method for aconnection structure for metallic members as recited in claim 7, whereinsaid solder consists of 94 to 98% by weight of Sn and 2 to 6% by weightof Ag.
 11. A connecting method for a connection structure for metallicmembers as recited in claim 7, wherein said solder contains at least oneof Zn, Cu, and Bi, in a total amount of 3% by weight or less.
 12. Aconnecting method for a connection structure for metallic members asrecited in claim 7, wherein said plated Sn—Zn alloy has a thickness of 3to 13 μm.
 13. A connecting method for a connection structure formetallic members as recited in claim 7, wherein said first metallicmember is a container and said second metallic member is a pipe.
 14. Aconnecting method for a connection structure for metallic members asrecited in claim 13, wherein said pipe is cooled by flowing coolant inthe inside thereof.
 15. A connecting method for a connection structurefor metallic members as recited in claim 13, wherein said pipe is cooledby spraying coolant in the external surface thereof.
 16. A connectingmethod for a connection structure for metallic members as recited inclaim 13, wherein said pipe is cooled by providing radiating members atan end thereof.